The present invention generally relates to actuators, and more particularly relates to a linear distance sensor.
It is known in the art to determine the position and angular speed of e.g. vehicle wheels by means of arrangements comprised of magnetically coded discs or rings and magnetic field sensors. For this purpose, permanent-magnetic material is fitted to the coded discs or rings (so-called encoders). A variation of the magnetic field on the encoder is required to have the position determined by the sensors. This is done either by alternating the north-south magnetization of the magnetic material along a circle arc on the encoder, or by a periodically reduced distance of the magnetic material from the sensor. The magnetic field is now scanned along the circle arc by a magnetic field sensor, which may e.g. be a Hall sensor or a magneto-resistive resistor, in order to determine the wheel position and the angular speed e.g. by counting the signal edges of the sensor signal. The signal produced by the sensor can be amplified and triggered by an electronic circuit fitted in the area of the sensor or integrated in the sensor (active magnetic field sensor).
A comparable arrangement is described in WO 95/17680, but also in WO 97/42508 for determining the wheel condition in a motor vehicle.
U.S. Pat. No. 4,712,083 discloses a magnetic distance sensor of high resolution which likewise uses the principle of the rotating permanent-magnetic encoder. The encoder is formed of evenly spaced hard magnets, and the magnetic field of all individual magnets is aligned towards the moving direction. The arrangement described is also provided exclusively to determine the position of rotating bodies.
To determine the slide controller position in a linear potentiometer, a slidable permanent magnet whose position is detected by means of a stationary gyromagnetic field sensor is used in DE 196 12 422. This type of sensor reacts especially sensitively to angular variations of the magnetic field.
A distance sensor to determine the position of a throttle valve in a motor vehicle is described in U.S. Pat. No. 5,929,631. In this arrangement, a Hall element is used as a magnetic field sensor or, what is preferred, a resistor element which makes use of the giant magneto-resistive effect (GMR). The said publication further mentions that the arrangement for position determination can be used for radial movements and for linear movements. The proposed solution resides in arranging a large number of magnetic sensors along the passage at a regular distance. In one embodiment, an arrangement is illustrated wherein eight GMR-sensors are arranged spherically on a rotating cylinder and, during the rotation, pass by a rod-shaped permanent magnet that is fixed to the inside of the cylinder jacket. This method of position determination is disadvantageous because a chain or matrix of individual sensors must be connected to an electronic evaluating circuit. Determining the position in this way is technically complicated and cost-intensive.
An increase in the resolution of magnetic distance sensors can be achieved according to DE 43 27 047 by anti-parallel aligning the hard magnets arranged on the encoder, on the one hand, and by employing two magnetic field sensors, on the other hand, which are arranged so as to be slightly offset laterally in the direction of the moving direction. The effect of outside disturbances can be further reduced by connecting several magneto-resistive deposit-film resistors to provide Wheatstone bridges. The arrangement described permits being generally used for measuring circular or linear changes in position of two objects that are movable in relation to each other. The above publication, however, does not provide any hints how a distance sensing unit must be constructed under practically relevant conditions as they prevail e.g. in motor vehicles.
In view of the above, an object of the present invention it to propose a linear distance sensor for motor vehicles which offers a maximum of reliability and a high distance resolution under conditions relevant in practice such as corrosion, wear, dirt, extreme heat, and extreme cold.
According to the present invention, this object is achieved by a linear distance sensor which is described in the following:
A linear distance sensor with an integrated magnetically effective component for e.g. mechanical actuation devices of brake units is described according to the present invention. Among others, the present invention is based on the idea of utilizing the mechanics of the actuation device additionally as the mechanics of non-contact linear distance indicators, the purpose of which is to depict the driver""s request for actuation either weighted distance-proportionally or distance-responsively.
The magnetic field of the encoder is measured or sensed by one or more magnetic field sensors. The magnetic field is sensed by the magnetic field sensor either completely or only partially, while it is to be understood by partial sensing of the magnetic field under the present invention that of the measurement quantity entirely describing the magnetic field, such as field strength and direction of the field vector, not all the quantities are sensed by the sensor module(s), but e.g. only the field strength and two direction coordinates of the field vector in the x-y plane of an appropriately chosen system of coordinates.
The sensor module according to the present invention comprises at least one magnetic-field-sensitive sensor and, if necessary, an electronic circuit for further processing the sensor signal. The present invention may be implemented by magnetic-field-sensitive sensors that operate according to the XMR principle, preferably, the AMR (Anisotropic Magneto-Resistive) principle, the GMR (Giant Magneto-Resistive) principle, or the Hall principle.
AMR principle means that the sensor utilizes the anisotropic magneto-resistive effect. Corresponding sensors are e.g. disclosed in S. Mengel, xe2x80x98Technologieanalyse Magnetismus Band 2: XMR-Technologienxe2x80x99, section 2.2, pages 18 to 20, VDI Technologiezentrum Physikalische Technologien, Dxc3xcsseldorf, 1997. GMR principle implies that the sensor element utilizes the xe2x80x98Giant Magneto-Resistive Effectxe2x80x99. The Hall principle implies that the sensor utilizes the Hall effect. Preferably, exclusively sensors are employed which operate according to the AMR principle.
In a favorable aspect, the linear distance sensor according to the present invention is characterized in that the sensor module comprises a bridge circuit made of magnetic field sensors whose main plane is aligned in parallel to the surface normal and to the longitudinal axis of the displaceable element.
Under the present invention, the surface normal of the displaceable element means a directional vector that is vertical to the surface of the displaceable element.
If e.g. the displaceable element assumes the shape of a rod with a circular cross-section, the surface normal corresponds to the radius vector of the rod.
In a second favorable embodiment, the main plane of at least one sensor module with bridge circuit is aligned vertically to the surface normal of the displaceable element.
It is especially advantageous when both above-mentioned sensor variations are realized in the linear distance sensor of the present invention. In this function principle, different magnetic field components of a magnetic encoder track are used, and the field strength pattern of the encoder track is converted into different signals.
The output signal or the output signals of the magnetic sensors that contain data about the movement are preferably provided in an electrical form at the output. This signal can be conditioned by one or more sensor circuits and e.g. made available in a digitalized fashion at the output of the sensor circuit.
The means for producing the permanent course of lines of magnetic flux is also referred to as encoder in literature. It comprises, for example, either a permanent-magnetic material which was alternatingly magnetized along its longitudinal axis or at least two series-arranged magnetized permanent-magnetic materials that modulate the course of the lines of magnetic flux by a different orientation or intensity of magnetization of the magnetic material. Bipolar or multipolar permanent magnets may be used, for example. Encoders are preferably used which comprise a homogeneous magnetic material which was magnetized corresponding to the desired course of lines of magnetic flux.
The permanent-magnetic material is aligned especially anti-parallel with respect to the magnetic north-south direction.
As permanent-magnetic material, e.g. permanent-magnetized ceramics material, for example, anisotropic barium ferrite magnets, is used, and plastics-bonded ferrite material is preferred. In a particularly favorable fashion, a material may be employed as a plastics-bonded magnetic material usable under the present invention which is utilized, for example, for manufacturing magnetized wheel bearing seals. This wheel bearing material is per se known and e.g. sold by the companies C. Freudenberg, Weinheim (Germany), SNR, Annecy (France), FAG Kugelfischer, Schweinfurt (Germany).
To improve the magnetic properties of the field-generating means, an iron return path may be provided on the backside. Preferably, an iron return path becomes unnecessary when permanent-magnetized ceramics material is used, however, it is expedient with plastics-bonded materials to arrange for a ferromagnetic return path. The iron return path is suitably composed of a magnetically conductive iron material that backs the field-generating means in the case of rod-shaped or ruler-shaped elements and forms a solid compound with it, what is especially preferred.
The shape of the field-generating means e.g. corresponds to that of a thin-walled tube, a narrow, flat ruler or that of a round rod. Preferred in use are field-generating means in the form of flat rulers with a spherical or trapezoidal profile, or round rods filled with an iron core.
The bearing guiding that guides in an axial direction is favorably so designed that the area of the field-generating means is sealed at least in part by the bearing itself. Additional sealing means in the area of the bearing becomes unnecessary in this case. It is especially advantageous when the sealing means itself performs the function of the bearing. The displaceable element is preferably comprised of a shank, an actuating element mechanically connected with the shank, and a force take-over means so that upon actuation of the actuating element by means of an outside force which acts upon the force take-over means, the shank and, thus, the field-generating means can be displaced axially in a way substantially free from tensile forces and/or pressure forces.
The displaceable element may have any cross-sectional shape desired and has either a massive design or an axial recess as it is e.g. provided in a tubular object.
Preferably, the displaceable element is a special section tube. According to the present invention, the term xe2x80x98special section tubexe2x80x99 refers to a conventional tube with any cross-section desired, e.g. round, oval, square, square with rounded edges, or polygonal.
The shank 7 may be of integral design or composed of several single parts. The connection between the shank and the actuating element is such that the shank is moved along with the rod in an exact manner in terms of position. Thus, the shank may be screwed to the actuating element.
Favorably, the cross-sectional shape of the shank generally corresponds to that of a special section tube as defined above, and the shank may additionally be hollow or solid. In a particularly favorable manner, however, the shank has an axial opening as it is typical of special section tubes.
The force take-over means for transmitting an outside force onto the actuating means e.g. serves to transmit the force of a brake pedal onto the brake cylinder. Preferably, the actuating means is a direct connection to the actuating rod of the brake cylinder or, in particular, is the rod itself. The force take-over means must generally be rated for forces in the direction of brake triggering, i.e., in the direction of the actuating rod, but it may also transmit forces under tensile stress, what is preferred. The force take-over means may be a rigid or movable connection to the brake pedal, preferably, a movable connection is concerned. The force take-over means is e.g. a ball-and-socket joint, a needle bearing, or a cone bearing.
The arrangement of the present invention for measuring linear distances favorably has a particularly low hysteresis.
A redundant arrangement according to the present invention refers to arrangements which are either fully redundant, partly redundant, or also comprise dually or multiply redundant systems so that in the event of failure or malfunction of a sensor or a sensor circuit, an element which exists twice will take over the function of the failing or faulty element, or detects malfunction thereof and, as the case may be, signals it to a monitoring device.
A specific resolution of the linear distance sensor is basically dependent on the qualities of the sensor and those of the field-generating means (encoder). For example, the variation of the magnetic field along the encoder (encoder track) can be varied by the distance of the single magnets, depending on the demands placed.
According to the present invention, linear distance-responsive magnetic codifications of the encoder track may be used, and also non-linear magnetic codifications. However, multi-track codifications may also be used, but preferably single-track codifications are employed, especially linear single-track codifications.
Another possibility includes designing the magnetic codifications so that, in interaction with appropriate sensors, an analog distance resolution is achieved which is much finer compared to a quantized distance resolution. In the practice, however, it is suitable to produce a quantization by a periodic repetition of a magnetization pattern so that a signal quantized in terms of distance prevails as an output signal.
The present invention also relates to the use of the above-described linear distance sensor for measuring the pedal position or lever position in an actuating device for brakes of motor vehicles.
Preferably, the linear distance sensor of the present inventions permits being used for piston shanks, actuating rods, throttle valves, and hydraulic pistons in motor vehicles.